RF plasma reactors of the type employed in processing semiconductor wafers require a large amount of RF power. Basically, the technique involves the ignition and maintenance of a processing plasma through the application of electric power to the plasma. The plasma interacts with gases introduced and with the target and wafer surfaces involved to affect the desired processing results.
Due to the increasing complexity of semiconductor devices, tighter and tighter control over the manufacturing process has been required. In order to achieve tighter process control in modern plasma processing, it is desirable to obtain more information about the associated RF voltage and current signals under actual processing conditions. This usually has been done by available V-I probes inserted in the power transmission path to measure the fundamental and harmonic signal power being directed to the plasma generation system.
Those skilled in the art have recognized that the fundamental and harmonic amplitude and phase angle relationships of the RF voltage and current signals account for much of the variation in process performance during semiconductor wafer manufacture. Due to non-linearity of the processing plasma, harmonics of the fundamental RF excitation frequency will be induced, even if the load appears to be matched at its fundamental frequency. As a result, the overall power delivered to the processing plasma includes the sum of the power levels of the fundamental and harmonic frequencies. Known plasma processing tools routinely employ two or more RF signal frequencies to enhance process performance yields. However, the introduction of two or more excitation frequencies into the plasma generation system tends to increase process uncertainty due to the introduction of intermodulation frequency components into the total power flow.
Prior art attempts have been made to characterize power flow in plasma processing, such as those disclosed in U.S. Pat. Nos. 5,523,955 and 5,273,610. For example, U.S. Pat. No. 5,523,955 discloses a measuring probe inserted in the power transmission path for sensing RF signals. The sensed signals are then used to indirectly derive AC signals for calculating phase angle information relating to the original sensed signals. However, until the present invention, the techniques required to directly measure the relative phase angle information of the fundamental and harmonic frequency content of the RF voltage and current signals in an accurate and stable fashion have not been readily available to those skilled in the art.
Therefore, there remains a strong need to provide a system and method for measuring and analyzing the critical amplitude and phase angle relationships between the fundamental signal frequencies and harmonics of the fundamental frequencies. Information characterizing the frequency content of the RF excitation signals can then be monitored to regulate and control power flow to the processing chamber in order to improve manufacturing yields, and make plasma processing more controlled and repeatable.
Although the present invention is described herein in terms of a system and method for analyzing power flow in semiconductor plasma generators, those skilled in the art will appreciate that the present invention may also be used in a variety of other power transmission systems including, but not limited to magnetic resonance imaging (MRI) systems and industrial heating systems such as inductive and dielectric heating systems. For example, in MRI systems, analysis of harmonic amplitude and phase information may be utilized to control and regulate magnetic resonance of transmitted signals under various load (e.g. patient) conditions. In industrial heating applications, analysis of harmonic amplitude and phase information may be utilized to control and regulate power flow to the work-piece and/or processing apparatus to improve processing performance.